Do you understand why the x' axis looks the way it does? If you do, you can use the same argument to see that every line that's parallel to the x' axis is a simultaneity line for the primed system, i.e. every point on such a line is assigned the same time coordinate by the primed system.

Now imagine one of those lines drawn between B and C. Points on the line have the same t' coordinate, let's say t'=T. Points above it (like C) have t'>T and points below it (like B) have t<T. Actually, you can see this more clearly if you draw two lines parallel to the x' axis that go through the points B and C. Since these are simultaneity lines, the points where they intersect the t' axis are assigned the same t' coordinates as B and C, and since those points are on the t' axis, you can see what their t' coordinates are just by looking at the scale drawn on the t' axis.

So because the simultaneity line through B hits the ct' axis earlier that C, it occurs first? And to determine this, the lines must be drawn parallel to the x' axis or ct' axis, depending on whether I'm trying to find when or where an event occured?